The obvious alternative is that it is a verb, a past participle to be precise. One property of participle verbs is that, when they function as adjuncts, they take the subject of the main clause as their subject. When people overlook the requirements, you end up with a silliness like the following (from here):
1 Now 83, and long gone from power, Britons remain fiercely divided over the reign of former Prime Minister Margaret Thatcher.
Notice that we expect this to mean that Britons are 83 and long gone from power, because Britons is the subject of the main clause. When the main clause has a dummy subject, like there, again we end up with problems (from here).
2 Fearing a massive lay-off, there was a general sense of relief when the boss announced there would be no new budget cuts.But when we replace the participle with a preposition, everything is fine.
3 After a massive lay-off, there was a general sense of anger.Our second piece of the argument is that, despite traditional thinking to the contrary, prepositions do not actually need to come before nouns. They can come before other prepositions, as they do here:
4 She came in from out of nowhere.Now, we can understand why compared is a preposition. Consider the following sentence:
5 Compared to ICS alone, there was a significantly greater improvement in FEV1 with the addition of LABA.To be honest, I don't know what it means, but it seems to me that there is no problem here analogous to what we faced in 2 even though we have a dummy subject. That means compared is not a verb. Perhaps, then, it could be a preposition along the lines of after in 3 and followed by another preposition to as in 4.
There are other prepositions like this: